Fuel cells are a power generating system for reacting a hydrogen-containing fuel gas with an oxygen-containing oxidant gas, i.e., causing an inverse reaction of the electrolysis of water on electrodes including a catalyst, to generate heat and at the same time electricity. This power generating system is high efficient and has features of low environmental loads and low noises, etc. as compared with conventional power generation methods and has been receiving attention as a clean energy source in the future. There are some types depending on the kinds of ionic conductors to be used and a type that uses a proton conducting polymer membrane is called a polymer electrolyte membrane fuel cell.
Polymer electrolyte membrane fuel cells among fuel cells are usable in the vicinity of room temperature, so that their use for vehicle power sources and stationary power supplies for homes has a bright prospect, and a variety of researches and developments have been going on recently. The polymer electrolyte membrane fuel cell is a cell sandwiched with a pair of separator plates in which gas passages for supplying a fuel gas containing hydrogen to one electrode and an oxidant gas containing oxygen to the other electrode are formed in an assembly produced by placing the pair of the catalyst layers on both sides of a polymer electrolyte membrane called a membrane and electrode assembly (hereinafter, may be abbreviated as MEA). Here, the electrode to which the fuel gas is fed is called anode and the electrode to which the oxidant is fed is called cathode. These electrodes include a catalyst layer produced by laminating carbon particles carrying a catalyst material such as a platinum-based noble metal to a polymer electrolyte and a gas diffusion layer having both gas permeability and electron conductivity.
Here, approaches that improve gas diffusibility have been performed on the catalyst layer in order to improve the output density of the fuel cell. Fine pores in the catalyst layer are located ahead through the gas diffusion layer and serve as a passage that transports a plurality of substances.
The anode plays the roles of not only smoothly feeding a fuel gas to a three-phase interface, which is an oxidation-reduction reaction field, but feeding water for smoothly conducting generated protons within the polymer electrolyte membrane.
The cathode plays the roles of feeding an oxidant gas as well as smoothly removing the water generated by electrode reaction. Techniques that improves drainage have been developed to prevent stopping of the power generation reaction due to the obstruction of mass transport, i,e., a phenomenon so-called “flooding”. (See, for example, Patent Documents 1 to 4)
The greatest challenge is to lower the cost although subjects for the practical use of polymer electrolyte membrane fuel cells include, for example, improvement of the output density and durability.
One means for lowering the cost includes the reduction of the humidifier. Though a perfluorosulfonic acid film or a hydrocarbon film is widely used for the polymer electrolyte membrane located in the center of the membrane and electrode assembly, moisture control near a saturated water vapor pressure is needed to obtain excellent proton conductivity, and water is supplied from the outside using a humidifier presently. Hence, the development of polymer electrolyte membranes is in progress that exhibit sufficient proton conductivity by low humidification and that do not need a humidifier, for low power consumption and the simplification of the system.
However, in a catalyst layer improved in drainage, the polymer electrolyte is dried up in low humidified conditions, whereby there is a need to optimize the catalyst layer structure and improve water holding properties. Up to now, for example, a method of sandwiching a humidity controlling membrane between the catalytic electrode layer and the gas diffusion layer is devised for improving water holding properties of the fuel cell in low humidified conditions.
In Patent Document 5, a method is devised in which a humidity controlling membrane made from a conductive carbonaceous powder and polytetrafluoroethylene offers a humidity control function and prevents drying up.
In Patent Document 6, a method is devised in which a groove is disposed in the surface of the catalytic electrode layer that makes contact with the polymer electrolyte membrane. The method is devised in which the groove having a width of 0.1 to 0.3 mm is formed to thereby suppress a decrease in power generation performance in low humidified conditions.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-120506    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-332041    Patent Document 3: Japanese Patent Application Laid-Open No. 2006-87651    Patent Document 4: Japanese Patent Application Laid-Open No. 2007-80726    Patent Document 5: Japanese Patent Application Laid-Open No. 2006-252948    Patent Document 6: Japanese Patent Application Laid-Open No. 2007-141588